1. Field of the Invention
The present invention relates to steels for high-strength springs excellent in cold workability and quality stability. More specifically, it relates to steels for high-strength springs which have excellent cold workability and highly stable quality after quenching/tempering in the manufacture of springs. The steels for springs according to the present invention can be used in manufacturing of steel parts in various fields such as the fields of transport machines such as automobiles and ships, and of industrial machines. The steels for springs will be described by taking automobile parts as a representative example of application.
2. Description of the Related Art
Demand for higher strength is made on steel materials constituting automobile parts such as valve springs and suspension springs accompanied with demand for reducing the size and thickness of automobile parts in order to reduce the weights of automobiles. The higher strengths of steel materials are generally achieved by increasing the contents of alloying elements. Such increased contents of alloying elements, however, reduce workability in the manufacture of springs. General manufacturing processes of springs are shown below. The increased contents of alloying elements of steel materials may often invite breakage during drawing and scalping processes for adjusting wire diameter and/or removing surface defects (hereinafter these processes are referred to as “pre-working processes” in contrast to a spring-coiling process.
<Manufacture of Hot-Coiled Springs>
A steel for springs is sequentially subjected to an acid pickling and coating process, a drawing and scalping process, a heating process, a hot coiling process, a quenching process, a tempering process, a prestressing and shot peening process, and a coating (painting) process and thereby yields a product spring.
<Manufacture of Cold-Coiled Springs>
A steel for springs is sequentially subjected to an acid pickling and coating process, a drawing and scalping process, a heating process, a quenching process, a tempering process, a cold coiling process, a annealing process, a prestressing and shot peening process, and a coating (painting) process and thereby yield a product spring.
As possible solutions to these problems, Japanese Laid-open (Unexamined) Patent Publication (JP-A) No. 2004-263247, for example, discloses a technique for preventing breakage during drawing process by controlling the contents of alloying elements to thereby control the hardness of a rolled steel. JP-A No. 06-184697 discloses a technique for ensuring cold workability by controlling component segregation of a steel.
Spring parts for use in automobiles are important parts from the viewpoint of safety and must maintain high quality over a long term to avoid damages such as chipping. To satisfy this requirement, steels for springs must maintain excellent properties even after quenching/tempering in the manufacture of springs. The conventional techniques, however, the stability of quality after quenching/tempering is not sufficiently considered, although improvements in processability in drawing are considered.